With availability of fossil fuels in decline, wind power is gaining greater acceptance as a viable source of energy. Rapidly increasing cost of energy and shrinking supply of conventional carbon fuels increases the urgency to improve wind power technology.
Wind turbines are generally divided into two broad categories in terms of the orientation of the axis upon which the turbine blades rotate namely horizontal axis wind turbines (HAWT) and vertical axis wind turbines (VAWT). HAWT is by far the more commercially successful category, principally since its proponents tout its advantages of efficiency, scalability, and power output. Advances in materials technology enables construction of large turbines with rotor diameters exceeding 100 meters, and power output of 2500 kw or more. Arguably, technical superiority of this class of wind turbines is attributable largely to the use of large, highly efficient axial flow airfoil rotor blades, whereas VAWT have primarily employed cross-wind, or radial flow drag type rotor blades of various kinds, with their associated disadvantages.
Airfoil blades are designed to maximize the lift to drag coefficient for high speed rotation. In the HAWT class, blades are aligned to receive air flow full face on the plane of rotation. When wind blows into the face of these turbine blades in an axial direction, all blades are ideally exposed simultaneously to air flow through the swept area of the turbine blade structure on the entirety of each blade from the tip to the root. Power is drawn from the wind passing through the swept area of the turbine rotor over the leading edges of blades, creating negative pressure on the “lift” surface. The total amount of power that can be extracted is dependent on overall diameter of the blade structure, blade configuration, and wind velocity.
Airfoil blade technology is advanced and reliable. Yet despite their technical superiority, HAWT have fallen from grace among many people in recent times. Its detractors claim HAWT have serious drawbacks in that they pose great peril to many avian species due to the location along migration routes and large diameter of the swept area of the turbine blades. Located near residential areas, the turbines are attributed to cause health problems in humans due to low frequency sound waves emanating from the exposed blades. For these reasons they are viewed by many as environmentally and socially irresponsible approaches to application of wind power. Such turbines are prohibited from many populated areas, including large urban centers, where the need for electrical energy is greatest.
On the operational side, some researchers claim serious drawbacks in HAWT wind farms, where multiple turbines require large distance separation in order to avoid the “wake” effects of turbulence from adjacent turbines, thereby reducing wind energy availability. Consequently, vast areas are needed for situating HAWT wind farms, removing the land from other uses. Wind farms must be located in remote areas, requiring costly electrical grids to transport electrical capacity to populated areas.
Researchers have debated the applicability of the Betz efficiency calculation as a valid standard of comparison for all types of wind turbines. It has been proposed that a more equitable approach is to use calculations of relative power output over given time periods as measures of comparison. VAWT devices fare much better when such calculations are applied, in particular when comparing relative power output in wind farms of comparable size.
VAWT technology has unrealized potential worthy of exploration. Vertical axis wind turbines have the advantage of potentially avoiding the need for realigning the turbine rotor to face directly into the wind. While they vary greatly in technical detail, they have certain things in common. Cross-wind, or radial flow turbine rotors receive air flow laterally in relation to the direction of rotation, and blades, or vanes rotate in opposition to wind flow during a portion of the rotation cycle. In order to improve performance, it is common to provide additional means or augmentations to counter the forces of wind flowing in opposition to blade rotation. Turbines such as those equipped with cupped blades or “scoops”, where the vertical dimension of the scoop often exceeds the rotor diameter, are examples of radial flow turbines. They rely on a surplus of positive air pressure or “drag” within a cupped configuration of the “scoop” moving in the direction of the wind to overcome the negative force of wind flowing against the outwardly facing surface of the rotating “scoop” moving in the direction opposing the wind. Power output is determined in part by the overall size of each “scoop” for collecting air flow, and effectiveness of means to counteract opposing force on the windward side of blade rotation.
Some augmented VAWT designs rely on various structural means for redirecting airflow to avoid the inefficiencies of blades rotating openly against the wind direction. These may include devices variously termed as shrouds, cowlings, diffusers, stators or other means to redirect horizontal wind flow into vertical streams capable of acting upon the rotor blades.
For example, U.S. Pat. No. 7,189,050 discloses a cross-wind turbine that includes a rotor with cupped drag type blades and vertically oriented airfoil stators for creating a low pressure area on the leading face of the rotor blade during a power stroke.
Such devices may use various means to cover portions of the structure where blades or paddles are moving in opposition to the wind direction. Part of the energy is thereby lost or unavailable for extraction by the turbine, which makes the cross-wind turbine a less efficient mechanism overall.
An attractive alternative to the disadvantages of radial flow or cross-wind VAWT machines is a type of VAWT using axial flow turbine blades similar to those used in HAWT, with the axis of rotation positioned vertically. This requires some means for redirecting horizontal wind flow vertically in order to propel the turbine rotor.
Various developments are disclosed in the prior art directed to VAWT devices employing axial flow turbine blades. A number of developments employ air intake openings which are shaped to define venturi passages for accelerating airflow. Common to such devices are fans, rotors and the like, driven directly or indirectly via redirection devices by means of air flowing from the discharge opening of the venturi passages entering axially into chambers housing the fan or rotor. Considerable attention is given to efficient design of the redirecting devices and means for concentrating airflow in an axial direction, and avoiding losses due to leakage and other inefficiencies.
U.S. Pat. No. 4,164,382 discloses a fixed axis turbine supported centrally of a fixed air guide defining a plurality of horizontal air passages disposed circumferentially of the turbine and each being adapted to receive an air stream therein coming from a limited range of wind directions and effect accelerated air flow toward the turbine.
Such efforts at times lead to intricate, costly designs requiring highly specialized tooling and fabrication techniques that present challenges to production for widespread distribution of the devices.
For example, in U.S. Pat. No. 7,400,057 an omni-directional, vertical discharge turbine has a shroud for capturing wind and directing it through a throat section where an aerofoil multibladed rotor is mounted. The intake of the shroud incorporates multiple horizontally curved blades of toroidal form varying up to nearly twice the size of the rotor diameter, stacked, staggered and secured in place by multiple aerodynamic vertical walls.
In the case of omni-directional devices in this class, a possible inefficiency comes from air circumventing the swept area of the turbine rotor and consequent loss of a portion of the available power to drive the fan. Proposed solutions include rotating the entire apparatus to face incoming wind.
Other possible solutions involve using movable blocking devices or gates to block air from leaking past inactive sectors, or multi-stage diverters or blocking devices for redirecting wind toward an axial flow type rotor.
U.S. Pat. No. 8,128,337 provides a two-stage, omni-directional vertical axis wind turbine in which wind exiting from a first radial flow turbine rotor passes through a diverter with radially aligned vanes to rotate an axial flow type fan. In the latter disclosure, ambient wind is redirected into vertical air flow through the swept area of the turbine blade structure more or less uniformly over the entirety of the rotor diameter.
Notably, the intent generally is to maximize pressure differential between the inbound and outbound bases of the swept area, and the airflows are generally concentrated within chambers enclosing the rotor and are uniformly distributed across the inbound bases of the swept area of the axial flow rotors.
Axial flow type rotors which rely on secondary airflow redirected via diverters or through venturi openings lose considerable portion of available power when the secondary airflow is dispersed over the swept area of the turbine rotor. Lacking in the prior art are means to translate generally horizontal wind flow into modulated air streams that impact airfoil blades selectively in close proximity and capable of effectively utilizing these modulated air streams to drive a vertically aligned axial flow rotor, in order to improve the utilization of airflow distributed across the swept area of the turbine blade structure.
It is required in the present invention that the turbine rotor be an axial flow rotor mounted for rotation about a vertical axis. None of the developments in the prior art disclose or contemplate augmented or modulated power extraction by means of stationary vanes aligned over the inbound face of the swept area of an axial-flow turbine rotor so as to focus high intensity air streams upon the leading edges of blades in a continuous progression to optimize energy extraction from ambient wind over a variety of wind conditions.
In assessing applicability of prior developments affecting the scope of the present disclosure, it is noted that the principal requirements for operability of the invention disclosed herein cannot be employed in, and therefore are not present in, horizontal axis wind turbines and vertical axis wind turbines utilizing cross-wind or radial flow rotors.
In the case of horizontal axis wind turbines, these devices are adapted to be positioned facing wind direction, such that airflow is delivered into the swept area axially. Augmentations such as shrouds surrounding the rotor can be used to concentrate airflow upwind or downwind so as to improve pressure differential. There are no known means by which the present invention may be used to augment power extraction in horizontal axis wind turbines wherein the rotor is positioned facing wind direction. Positioning the rotor so as to receive airflow laterally would defeat the intent of the horizontal axis wind turbine and produce results of questionable value. In the case of wind turbines with radial flow rotors, and in particular vertical axis wind turbines with radial flow rotors, such devices are known to use stators or vanes to redirect wind into the rotor, however, their structure and operation are substantially distinct from the present invention.
It is known that force exerted laterally on an axially rotatable object delivers torque proportional to the length of the lever arm multiplied by magnitude of the force. By extension, a force applied on a turbine blade near the tip delivers greater torque over a narrower arc than an equal force exerted closer to the blade root. Using these concepts, it is possible to modulate or manipulate airflow distribution across the swept area of the turbine blade structure to optimize energy extraction.
The present invention proposes focusing a portion of an intensified air stream upon the leading edge of an axial flow type airfoil blade for the duration of its movement through an arc, such that the combined forces impinging on the blade deliver available force selectively, and thereby more effectively than if the entire air stream is distributed uniformly across the swept area of the turbine blade structure. No attempt is made in the prior art mentioned above to utilize this concept in a manner disclosed herein to optimize power extraction from axial flow turbine rotors over a variety of wind conditions.
The present invention is intended to provide an effective approach for augmenting the power extracted from a vertically mounted axial flow rotor.
It is a primary objective of the present invention to provide a more cost-effective means for generating electrical energy in a variety of wind conditions, utilizing the herein disclosed improvements in vertical axis, axial flow turbines.
Another objective is to provide a wind turbine design suited for installation in urban settings, such as building rooftops, utilizing the herein disclosed improvements in vertical axis axial flow turbines.
Another objective is to provide a scalable wind turbine design suited for use by public utilities, communities, corporations, and individual homeowners.
Another objective is to provide a relatively low-cost, simple design using readily available materials and thereby reducing some of the barriers to acceptance and wide-spread adoption of vertical axis wind turbines.